Articulations of the human body are often very complex systems and no precise generic model exists to capture all the variability from one articulation to another. It is therefore necessary to use specific medical images or collection of digital patient data in order to get relevant information to develop techniques, devices and methods that will facilitate a treatment or a diagnosis.
In a specific example related to the hip articulation, structural abnormalities in the morphology of the hip can limit motion and result in repetitive impact of the proximal femoral neck against the acetabular labrum and its adjacent cartilage. Femoro Acetabular Impingement (FAI) is a pathology that can result from a decreased femoral head-neck offset (cam effect), an overgrowth of the bony acetabulum (pincer effect), excessive acetabular retroversion or excessive femoral anteversion, or a combination of these deformities. The cam impingement is generally characterized by a bone overgrowth located at the antero-superior aspect of the femur head-neck junction, which destructures the spherical shape of the femur head. The pincer impingement is generally characterized by an overcoverage located at the anterior aspect of the acetabulum rim. However, the correct and full diagnosis of this pathology is not easy to determine, especially when dealing with subtle deformities. Standard radiographic X-rays are used for the initial diagnosis and then three dimensional (3D) Computed Tomography (CT) scans or Magnetic Resonance Imaging (MRI) exams are generally performed in case of suspected FAI pathology. The processing of the 3D images remains a laborious manual task which cannot ensure accuracy and reproducibility, potentially misleading the diagnosis or the surgical indication. Moreover, even though 3D information can be extracted from such exams, the reconstructed bone volumes remain static and cannot predict with reliability the exact location of the impingement which occurs during the mobilization of the hip.
The surgical treatment of FAI aims at restoring a normal spherical shape to the femur head neck junction at the level of the bony cam lesion and restoring a normal coverage rate of the acetabular rim at the level of the pincer lesion, by removing the excess of bone. The result of this bony reshaping is the restoration of a greater range of motion of the hip, without impingement. Conventionally, the open surgical approach had initially been adopted since it provides a full exposure of the bone and direct access to the anatomy to be treated. Though, since minimally invasive procedures have grown in popularity by reducing the pain, morbidity and recovery time for patient, arthroscopic treatment of FAI has been explored in the last decade, which requires the use of an endoscopic camera and specific small instruments that can pass through various types of canulas. Advantages include minimally invasive access to the hip joint, peripheral compartments, and associated soft tissues. Furthermore, arthroscopy allows for a dynamic, intra-operative assessment and correction of the offending lesions. However, due to the depth of the joint and the reduced visibility and access, theses hip arthroscopy procedures are difficult to perform and not all surgeons feel comfortable about adopting the technique. The success of such arthroscopic interventions relies on correct diagnosis, accurate pre-operative assessment of the pathology, very meticulous intra-operative evaluation and a thorough and accurate correction of impingement lesions on both the femoral and acetabular sides, which can only be accomplished after a laborious learning curve over many cases. Failure of arthroscopic procedures for FAI is most commonly associated with incomplete decompression of the bony lesions.
Hence, one important issue is the difficulty to determine precisely and in a reproducible manner the location and amount of bone to be resected on a deformed articulation bone surface in order to recreate a smooth bone surface. The surgeons are generally applying 2D templates over the patient X-ray images to try to estimate the resection to be achieved. This remains a very limited and inaccurate method for addressing a problem in 3D space. The acquisition of a pre-operative 3D image of the patient is becoming a common protocol in these pathologies, thus increasing the level of information of the surgeon on the pathological problem. However, there are very few tools to process these 3D images and use resulting information in order to provide a proposition for the bone correction to be performed. Most of the imaging systems used to acquire the 3D images provide 3D reconstruction of bone surface models, however, the processing have to be applied manually and the results are only static projection views of the bone models. There exists some software proposing to simulate the resection pre-operatively, like the Mimics® software from Materialise, Leuven, Belgium, but the tools they offer are only simulation of bone milling process to be applied manually by the user, point by point, which takes a lot of time to perform, and does not guarantee reproducible results based on objective criteria. Another method consists in using the opposite side of the patient and mirror the opposite surface to define an optimal correction surface on the deformed side, but accurate results cannot be provided if the opposite side has also some early stage of deformity.
The characterization of the bone deformation by a so-called “alpha angle” measured on slice of the 3D image passing by the neck axis and quantifying the bump deformation on the head neck junction by a deviation measure from an ideal sphere has been described by Notzli et al (2002). Some methods have been developed to determine the resection to be applied to correct the deformation by removing the excess of bone which deviates from the ideal sphere (Kang et al, 2005 and Tannast et al, 2006).
However a precise parameterization of the boundary of the targeted correction and the shape of the corrected bone surface has not been provided yet. One difficulty is to minimize the number of parameters defining such correction while ensuring to provide a valid correction covering individual specificities of the deformation.
In particular, obtaining a smooth transition and a minimal indentation for the new shape of the bone after correction has been formulated by several authors as reasonable and obvious criteria, but no method for efficient routine use has been proposed.